Time Division Duplex (TDD) is a transmission mode supported both in 3GPP UMTS, LTE (Long Term Evolution) and IEEE 802.16 that utilizes the same radio access scheme as the Frequency Division Duplex (FDD), i.e. in case of LTE OFDMA (Orthogonal Frequency-Division Multiple Access) in the downlink and the SC-FDMA (Single Carrier Frequency-Division Multiple Access) in the uplink, CDMA in case of UMTS and OFDMA in case of IEEE 802.16 in both uplink and downlink. Furthermore, TDD uses the same subframe format as well as the same configuration protocols as FDD. The main difference compared with FDD is that TDD macro cellular base stations, or evolved Node B (eNBs) in 3GPP terminology, support an unpaired frequency band, where downlink and uplink are separated in time domain, with each frame being composed by downlink (DL), uplink (UL) and special (S) sub-frames.
Special sub-frames are used to switch from downlink to uplink and they are included at least once within each frame. In particular, the special sub-frame consists of the following three special fields, a downlink part (DwPTS), a guard period (GP), and an uplink part (UpPTS). In 3GPP LTE, the UL/DL portion of each frame may be configured according to the specification provided in document 3GPP TS 36.300, Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Rel 10), April 2011, which defines 7 different UL/DL configuration modes as shown in the following table:
Uplink-downlink allocationsConfig-Switch-pointSubframe numberurationperiodicity012345678905 msDLSULULULDLSULULUL15 msDLSULULDLDLSULULDL25 msDLSULDLDLDLSULDLDL310 ms DLSULULULDLDLDLDLDL410 ms DLSULULDLDLDLDLDLDL510 ms DLSULDLDLDLDLDLDLDL65 msDLSULULULDLSULULDL
Despite the provision of flexibility in resource allocation, such asymmetric UL/DL brings new challenges in admission control and load balancing. The main difference from the conventional FDD resource allocation is the fact that in TDD it is a slot specific process, with each slot being considered individually. Certain overloaded slots may introduce congestion for particular applications with specific UL/DL-requirements regardless of resource availability on different slots. Such phenomenon is referred in literature as “pseudo congestion,” because an eNB or cell seems congested without its resources being fully utilized.
A simple example to demonstrate “pseudo congestion” is illustrated in FIG. 1, where User Equipment UE A, served by eNB A, with high DL demand creates and experiences congestion due to the restricted availability of DL resources of eNB A, while the same situation accounts for UE B, served by eNB B, with high UL demands due to low UL resource availability of eNB B. It should be noted that in both cases the associated eNBs are not fully overloaded since they have available resources, eNB A in the uplink and eNB B in the downlink.
Initial efforts towards Call Admission Control (CAC) solutions for TDD systems that consider such mismatch of UL/DL resource allocation concentrate on decoupling the problem into two independent ones that focus on the UL and DL, with each CAC module employing a particular policy separately. Such approach is described, for instance, in B. Rong, et. al, “Call Admission Control Optimization in WiMAX Networks”, IEEE Transactions on Vehicular Technology, Vol. 57, No. 4, July 2008. Connection requests with a traffic demand that fulfills both UL and DL CAC are permitted to be established. Such a policy realizes the significance of asymmetric UL and DL treatment within CAC, which is also important for the process of load balancing in TDD and “pseudo congestion.”
TDD specific load balancing shares similarities with conventional cell breathing, as described, e.g., in US 2009/0323530 A1. Cell breathing refers to the process which regulates the number of serving users per cell by increasing or decreasing cell size or coverage area. TDD load balancing also adopts cell size and traffic concentration regulation concepts but is based on UL and DL separately compared to conventional cell-breathing. The non-separated treatment of UL and DL is the main cause of the “pseudo congestion” problem. Therefore, the load balancing problem in TDD has two dimensions, named the load variation among neighboring cells and the variation among UL and DL or other levels of asymmetry, as described in M. Peng, W. Wang, “A Framework for Investigating Radio Resource Management Algorithms in TD-SCDMA Systems”, IEEE Communication Magazine, Vol. 43, No. 6, June 2005.
Previous efforts to resolve the “pseudo congestion” problem concentrate on heterogeneous environments where base stations and Wi-Fi access points operate in TDD, as described in W. Weidong, et. al., “A Call Admission Control Algorithm based on Inter-Link Load Balance in Heterogeneous Networks”, IEEE IC-BNMT Beijing, October 2009. The focus is on CAC and on specific algorithms based on the notion of inter-link load imbalance, which refers to the difference in resource utilization among UL and DL of a single base station or access node. The aim is to keep such inter-link load imbalance as low as possible by the means of matching the user demands with the system resource availability. In particular, a user assigning algorithm identifies the appropriate base station or access point with the resource availability closer to the particular application characteristics in terms of UL/DL demand. In other words, the CAC algorithm chooses the attachment point or network that produces the smallest imbalance distance between its UL and DL resource availability.
An alternative approach that treats the UL and DL channels independently with respect to the cell coverage and the corresponding geographical area has been described in K. Mori, et. al, “Asymmetric Traffic Accommodation using Adaptive Cell Sizing Technique for CDMA/FDD Cellular Packet Communications”, IEICE Trans. Fundamentals Vol. E90-A, No. 7, July 2007. The aim is the provision of asymmetric UL/DL resource allocation with the objective to match the capacity offered by operators to the traffic demand for every cell within the Radio Access Network (RAN). The means of such asymmetric UL/DL resource allocation is to configure independently for UL and DL channels its coverage size jointly with neighboring cells. Thus, the UL and DL cell coverage may be different which provides a higher degree of resource flexibility. This allows load balancing not only on conventional cell breathing basis but additionally based on each particular channel load in relation with the user's geographical position and cell load conditions. A simple example that demonstrates the main concept is illustrated in FIG. 2, where eNB A accommodates, compared to eNB B, a higher density of DL users than UL users.
Although the prior art solutions described above manage to solve the “pseudo congestion” problem, they are disadvantageous in that they prove to be rather inflexible with respect to an adaptation to individual UE UL/DL demands.